This invention pertains to a transmission having a hydrostatic unit and which includes lubrication and charge circuits, with means to modify the operation of the transmission to prevent damage thereto upon malfunctioning of components which could affect the pressure in either of the lubrication or charge circuits.
A primary feature of the invention is to provide suitable controls for a transmission having a hydrostatic unit and lubrication and charge circuits to: assure that pressure in the lubrication circuit does not drop below a safe level; reduce the control pressure for the hydrostatic unit to provide for sluggish operation thereof if pressure in the charge circuit goes below a certain level; increase charge pressure as a function of the control pressure for the displacement control of the hydrostatic unit to assure adequate clutch pressures in one or more clutches used in the transmission system; and to provide for an increase of pressure in the charge circuit during overspeed conditions of the hydrostatic unit to avoid damage thereto.
Another object of the invention is to provide a transmission having hydraulic components with a lubrication circuit and a charge circuit, first and second positive displacement pumps in series with the first pump having a larger volume than the second pump, said lubrication circuit and second pump being operatively connected to the outlet of the first pump, said charge circuit being operatively connected to the outlet of the second pump, a fluid line connected between the lubrication circuit and charge circuit, and means in said fluid line for blocking flow therethrough and operable in response to a predetermined drop in pressure in the lubrication circuit for opening said fluid line to permit fluid flow from the charge circuit to the lubrication circuit in order to maintain a desired pressure of fluid in the lubrication circuit.
Still another object of the invention is to provide a transmission as defined in the preceding paragraph wherein the means in the fluid line includes a lubrication relief and priority valve which selectively operates to permit fluid to flow from the charge circuit to the lubrication circuit and which also is responsive to the pressure in the lubrication circuit to limit the pressure in said lubrication circuit.
Still another object of the invention is to provide a transmission having a hydrostatic unit with a variable displacement component having a hydraulic displacement control and with a charge circuit having a pair of branch lines with one branch line connected to the displacement control and the other branch line supplying hydraulic components including make-up fluid for the hydrostatic unit, and a charge priority valve in said one branch line operable to reduce the pressure in said one branch line while maintaining the pressure in the other branch line to result in reduced available pressure for control of the displacement of the hydrostatic unit with resulting sluggish operation thereof when necessary to provide an indication of a problem existing in the transmission.
Still another object of the invention is to provide a transmission, as defined in the preceding paragraph, wherein the charge priority valve is responsive to a control pressure existing in a control servo of the displacement control whereby as said control pressure increases to indicate an increase in output torque of the hydrostatic unit, such control pressure modifies the action of the charge priority valve to increase the pressure in said other branch line in order to assure adequate pressure to hydraulic components, such as clutches connected to said other branch line.
A further object of the invention is to provide a transmission as defined above wherein said charge priority valve is also subject to pressures corresponding respectively to the speed of the hydrostatic unit and the speed of an engine, such as a vehicle engine connected to the transmission and which provides power input to the hydrostatic unit whereby when the hydrostatic unit reaches a speed equal to its overspeed rating, the charge priority valve is caused to operate to increase the pressure in said other branch line in order to meet the requirement that a higher charge pressure is required as a higher hydrostatic unit speed occurs.